In a system using an engine (e.g. an automobile or the like), a state determination, i.e. a diagnosis, of an exhaust system (an exhaust gas sensor or an exhaust gas purification catalyst or the like) is performed by an engine electronic control unit (hereinafter, referred to as “ECU”). This on-board diagnosis (OBD) of the exhaust system includes a catalyst temperature estimation, a catalyst malfunction diagnosis, an exhaust gas sensor malfunction diagnosis, which are explained below, etc.
(1) For example, in this kind of the system, a catalyst is positioned in an exhaust passage in order to purify an exhaust gas. Generally, the catalyst has a property that a purification ratio is high only within a prescribed temperature range (e.g. 400-800° C.). Accordingly, various proposals are conventionally made to increase the catalyst temperature rapidly after an engine is started (for example, Unexamined Japanese Patent Publication No. 2007-231820, etc.).
Further, this kind of the catalyst is deteriorated by deleterious components (lead and sulfur, etc.) in fuel and heat. When the catalyst is deteriorated, an exhaust gas purification ratio is decreased and an exhaust emission is increased. Accordingly, various kinds of devices for determining the deterioration of the catalyst are conventionally proposed (for example, Unexamined Japanese Patent Publication Nos. 5-133264 and 2004-28029, etc.).
Meanwhile, so-called three-way catalyst is widely used as this kind of the catalyst. The three-way catalyst has a function called as an oxygen adsorption function or an oxygen storage function. The function is one which reduces NOx (nitrogen oxide) in the exhaust gas and adsorbs (stores) oxygen removed from the NOx therein when an air-fuel ratio of air-fuel mixture is lean, while discharging the adsorbed oxygen for oxidizing unburned components such as HC and CO, etc. in the exhaust gas when an air-fuel ratio of air-fuel mixture is rich. Accordingly, as a maximum value (hereinafter, referred to as “maximum oxygen storage amount”) of an amount (hereinafter, referred to as “oxygen storage amount”) of the oxygen which can be stored by the three-way catalyst is large, a purification ability of the three-way catalyst is high. In other words, a deterioration state of the three-way catalyst can be determined by the maximum oxygen storage amount.
In a catalyst deterioration detection device disclosed in the Unexamined Japanese Patent Publication No. 5-133264, a first air-fuel ratio sensor is positioned upstream of the three-way catalyst positioned in the exhaust passage. Further, a second air-fuel ratio sensor is positioned downstream of the three-way catalyst positioned in the exhaust passage. In this configuration, a deterioration determination of the three-way catalyst (a calculation of the maximum oxygen storage amount) is performed as follows. First, an air-fuel ratio of an air-fuel mixture supplied into a cylinder of an engine is set to a predetermined lean air-fuel ratio for a predetermined time period. Thereby, the oxygen is stored in the three-way catalyst to an upper limit of the adsorption ability thereof. Thereafter, the air-fuel ratio of the air-fuel mixture is forcibly changed to a predetermined rich air-fuel ratio. Then, the air-fuel ratio detected by the second air-fuel ratio sensor is maintained to a stoichiometric air-fuel ratio for a constant time period Δt, and thereafter is changed to a rich air-fuel ratio. On the basis of the difference Δ(A/F) between the stoichiometric air-fuel ratio and the rich air-fuel ratio, Δt, and an intake air amount, the maximum oxygen storage amount is calculated.
However, the maximum oxygen storage amount changes depending on a temperature of the three-way catalyst. Specifically, when the temperature of the three-way catalyst increases, the maximum oxygen storage amount increases. Therefore, the catalyst deterioration determination which is performed on the basis of the maximum oxygen storage amount calculated without considering the catalyst temperature has a problem that the determination accuracy is not adequate. Accordingly, a catalyst deterioration detection device disclosed in the Unexamined Japanese Patent Publication No. 2004-28029 is configured to correct the maximum oxygen storage amount based on the catalyst temperature at the period of calculating the maximum oxygen storage amount.
As explained above, the catalyst temperature is an important parameter for the on-board diagnosis of the warm-up state and the deterioration state, etc. of the catalyst. The catalyst temperature can be measured by a catalyst bed temperature sensor (for example, see the Unexamined Japanese Patent Publication No. 2005-69218, etc.). Alternatively, the catalyst temperature can be estimated on board by using other engine parameters such as intake air flow rate, etc. (for example, see the Unexamined Japanese Patent Publication Nos. 2004-28029 and 2004-197716, etc.). In terms of responsiveness, accuracy, cost, etc., it is preferred that the catalyst temperature is estimated on board, rather than measured by a sensor.
(2) For example, in order to control an air-fuel ratio of an engine, a so-called air-fuel ratio feedback control is normally performed. The control is performed on the basis of an output of an exhaust gas sensor (an air-fuel ratio sensor) positioned in an exhaust passage. The exhaust gas sensor is generally an oxygen sensor for generating an output corresponding to an oxygen concentration in an exhaust gas. The exhaust gas sensor(s) is/are provided upstream and/or downstream of a catalyst for purifying the exhaust gas in the flowing direction of the exhaust gas.
The exhaust gas sensor provided downstream of the catalyst is normally comprises a solid-electrolyte type oxygen sensor which has an output property that an output is generally constant under a rich air-fuel ratio relative to the stoichiometric air-fuel ratio and under a lean air-fuel ratio relative to the stoichiometric air-fuel ratio and rapidly changes around the stoichiometric air-fuel ratio. The exhaust gas sensor provided upstream of the catalyst is normally comprises the above-mentioned solid-electrolyte type oxygen sensor or a limiting-current type oxygen concentration sensor which has a relatively linear output property within wide range of the air-fuel ratio.
When a malfunction occurs in the above-mentioned exhaust gas sensor, an air-fuel ratio control of the engine may not be appropriately performed. Accordingly, a device for performing a malfunction diagnosis of the exhaust gas sensor is conventionally proposed (for example, see the Unexamined Japanese Patent Publication Nos. 2003-254135, 2004-225684, 2007-16712, etc.).
This kind of the device is configured to determine if the exhaust gas sensor is normal on the basis of the response state of the exhaust gas sensor to the air-fuel ratio change of air-fuel mixture. For example, in a device disclosed in the Unexamined Japanese Patent Publication No. 2004-225684, the air-fuel ratio is forced to be alternatively changed between predetermined rich and lean air-fuel ratios, and it is determined if there is a sensor malfunction on the basis that whether a sensor output correctly follows the air-fuel ratio change.